Cast coating of paper and board



United States Patent 3,189,503 CAST COATING OF PAPER AND BOARD Frank D. Bergstein, Wyoming, and Alfred B. Kleingers,

Jr., Middletown, Ohio, assignors to Bergstcin Packaging Trust, a trust composed of Robert M. Bcrgstein and Frank D. Bergstein, trustees No Drawing. Filed Nov. 20, 1959, Ser. No. 854,240

Claims. (Cl. 156231) This invention relates to the manufacture of mineral coated paper and board wherein a wet mineral coating is dried in contact with a casting surface, such as a polished metal drum or endless belt, the resultant product when stripped from the casting surface exhibiting a mineral surface having a finish corresponding to the finish of casting surface and known in the trade as a cast coated product.

For many years such cast coated products have been manufactured by applying a wet or plastic mineral coating composition to one surface of a paper or board web, whereupon the wet coating is pressed against the casting surface and maintained in non-slipping contact therewith until the coating composition has thoroughly dried. Upon being dried, the coated web is stripped from the casting surfaces. In most instances, the casting surface comprises a highly polished chromium plated drum which is internal- 1y heated to effect drying of the mineral coating. Such chromium plated drum results in a cast mineral surface characterized by a high gloss finish.

One of the major problems encountered in cast coating is to obtain clean and complete release of the dried mineral coating from the casting surface. Unless the mineral coating is stripped cleanly, its gloss finish will be impaired. Similarly, if any portion of the coating composition sticks to the casting surface, it notonly mars the surface of the cast product but additionally and progressively builds up larger and larger deposits on the casting surface, with the result that the casting drum must be taken out of operation and its surface painstakingly cleaned to remove each and every particle of coating composition adhering thereto. Over the years various expedients have been suggested to effect clean release of the mineral coating from the casting surface. It has hitherto been suggested that clean release of the mineral coating could be obtained by applying a thin layer of oil or other oleaginous material to the casting surface, such layer acting as a parting film to permit the cast mineral coating to be cleanly detached from the casting surface. In accordance with the teachings of Montgomery Patent Re. 23,637, the addition to the coating composition of a small quantity of a saponifiable material, such as vegetable oils and fatty acids, result in the formation of a thin oily film on the casting surface which prevents the mineral coating from sticking to the casting surface. The Montgomery teachings are premised upon the fact that a saponifiable material incorporated in an alkaline coating composition will be at least partially saponified to produce soluble soaps which are reliable release agents and provide the desired oily parting film. The use of unsaponifiable release agents has also been suggested. Thus, Hart Patent 2,776,226 teaches the use of ketene dimers as a release agent, such ketene dimers being admixed with an emulsifying agent and incorporated in the mineral coating composition. However, it would appear that such emulsions also produce a parting film on the casting surface very much similar to the oleaginous film forming materials of the Montgomery patent, due in part at least to the presence of saponifiable oils in the emulsifying system.

In accordance with the instant invention, we have now discovered that mineral coating compositions can be compounded which are inherently self-releasing and do not require a parting film on the casting surface to effect clean ice release of the mineral coating. Extensive tests have established that mineral coating compositions formulated in accordance with the instant invention adhere tenaciously to the casting surface during the setting and drying of the mineral coating, and yet when the coating is stripped from the casting surface, it releases cleanly and completely, the stripped coating exhibiting a uniformly high quality high gloss cast finish. At the same time the casting surface itself remains completely clean and free from any residual oil absorbed from the coating composition. The techniques of the instant invention are highly advantageous in that the casting surface need not be pre-treated with a film forming material, as is required with prior art techniqucs, nor is it necessary to utilize alkaline coating compositions to effect saponification of the release agent. In fact, ithas been found that superior cast coatings can be formulated by working wholly on the acidic side and maintaining the entire composition on the acid side throughout the entire coating operation. The invention is nevertheless equally applicable to alkaline coating compositions.

The instant invention will find utility not only in the application of wet clay coated board to a casting surface in the manner hereinbefore described, but it will also find utility in the manufacture of cast coated board in accordance with the teachings of the copending application of Frank D. Bergstein, one of the co-inventors herein, Serial No. 638,680, filed February 7, 1957, and entitled, Dry Gloss Pre-Cast Clay Laminated Paper and Method of Making It, now Patent 2,934,467, which teaches a radical departure from prior art casting coating techniques wherein a thin uniform layer of mineral coating composition is pre-formed and dried against a heated casting surface and thereafter adhesively bonded to a web of paper or board,

-Whereupon the laminated structure is stripped from the casting surface. This technique has resulted in the production of cast coated boards and papers at very much higher speeds than obtainable where the mineral coating is first applied to the paper or board web, and at the same time the laminated construction permits a material reduction in the quantity of mineral coating required to provide uniform opacity together with enchanced printability. The interposed adhesive layer may be utilized as a means for providing proofing characteristics heretofore unattainable in conventional cast coated products.

Mineral coating compositions utilized for cast coating purposes-whether of conventional character or in accordance with the aforementioned Bergstein applicationare composed essentially of pigment and binder, the pigment generally comprising clay, calcium carbonate, titanium dioxide, or combinations thereof, and the binder comprising an adhesive such as casein, alpha protein, or a resin emulsion, such as acrylic polymer resin emulsion or a vinyl acetate emulsion. In compounding these materials, conventional mineral coating formulations may be employed, the pigment and binder ingredients being admixed to pro vide from 70-90 parts pigments to 30-10 parts binder on a dry weight basis. The composition may be readily compounded by dispersing the pigment in water to form a slurry of about 50-65% solids; and if casein or alpha protein are used as the binder ingredient, they will be first cut with ammonia water to about 1525% solids. In the case of resin emulsions, they may be used in the emulson form as supplied, such emulsions normally containing from 40-65% solids. In any event, the binder solution or emulsion will be added to the pigment slurry and sufiicient water added to bring the overall solids content to between -65%.

In accordance with the instant invention, it has been found that the mineral coating composition just described can be rendered self-releasing by the addition thereto of a relatively small quantity of a polyethylene emulsion,

. binder to form the coating slurry.

In compounding the mineral coating composition, it has becnfound that highly satisfactory self-release properties can be developed employing 6 parts of polyethylene emulsion to a coating composition having a total solids contents of approximately 120 pants, i.e., 100 parts pigment and par-ts binder. In a preferred formulation, the polyethylene may be compounded as follows:

Parts Emulsifiable polyethylene Polyoxyethylene lauryl ether 11.2 Sorbitan trioleate 0.8 KOH solution) 1.2 Water 150 Since the emulsion itself contains somewhat less than 20% emulsifiable polyethylene, this means that on a dry basis there is about 1 part of polyethylene solids per 120 parts pigment and binder solids. Yet the presence of this small quantity of polyethylene results in a coating composition which, when dried, can be readily and cleanly stripped from the casting surface.

It is interesting to note that the ingredients making up the emulsifying system of the foregoing example. which comprises polyoxyethylene lauryl ether,'sorbitan trioleate,

and potassium hydroxide, are not in themselves release agents for the coating composition when used in the quantities set forth. That is, neither the emulsifying system or any of its constituents are effective release of stripping agents when employed as such. Rather, it is the polyethylene which is dispersed throughout the coating composition which renders the coating self-releasing and not any release properties which are inherent in the emuliifying system and imparted thereby to the casting surace.

It is also interesting to note that the melting point of polyethylene is in the range of 215221 F., whereas the heated casting surface against which such coating is dried is normally maintained below 180 F. to prevent blistering and bubbling of the coating. Consequently, the polyethylene is not heated to a temperature at which it would liquify. It is believed that the polyethylene remains dispersed throughout the coating composition. The mechanism by means of which release is effected is not known, although we are satisfied that it is not by reason of the formation of a parting film at the interface of the casting surface and the mineral coating. Rather, it would appear that the release mechanism afforded by the polyethylene is wholly contained within the coating itself and results in a conditioning of the coating efi ective to permit the cast surface to release upon the application of stripping forces.

While a preference has been expressed for a coating composition containing 6 parts of polyethylene emulsion to 120 parts pigment and binder solids, we have found that as little as 2 to 3 parts polyethylene emulsion per 120 parts pigment and binder solids will be effective. Similarly, the amount of polyethylene emulsion may be increased well beyond 6 parts if so desired although it does not appear that the release properties are enhanced to any preceptible degree. Formulations containing 12 parts polyethylene emulsion provide excellent stripping properties; but these properties are not perceptibly improved over formulations containing a lesser quantity of polyethylene. Consequently, the quantity of polyethylene present in--the coating composition may be widely varied but preferably will be maintained relatively low for practical purposes.

We have found that an excellent coating composition results where the binder ingredient comprises a fine particle size polyvinyl acetate resin emulsion, such as Dewey & Almys Darcx 63-M. This emulsion is on the acidic side and best results are obtained where the composition is maintained on the acidic side by employing a polyethylene emulsion which is compatible with the acidic binder. The aforementioned emulsifying system incorporating as its principal constituent a non-ionic emulsifier such as polyoxyethylene lauryl ether producesexcellent results. Other emulsifying systems may also be employed. For example, a highly satisfactory emulsifying system may be compounded using a commercial emulsifying agent known as Hyonic FA-40, manufactured by Nopco Chemical Company and comprising a coconut oil' alkylolamide with added fatty acid diethanolamine soap. Such emulsifying agent is preferably used in conjunction with a small quantity, such as about 1%, of a suitable defoamer.

While we have expressed a preference for coating compositions wherein the binder ingredient comprises a vinyl acetate emulsion, such as the aforementioned Darex 63-M, we have found that polyethylene atfordsexcellent self-release properties in coating compositions employing other types of binder. Thus, styrene-butadiene emulsions may be employed, or the binder may be composed of an acrylic polymer, such as Rhoplex B-'60K manufactured by Rohm and Haas. Similarly, natural binders such as alpha protein and casein may be employed in conventional formulations. While coatingcompositions which are on the alkaline side tend to saponify any fats or oils which might be present in the emulsifying system, such reactions are purely incidental and immaterial to the practice of the instant invention.

It will be understood, of course, that coating compositions formulated in accordance with our invention will be cast against a heated casting surface, such as a revolving drum, and dried in situ thereagainst, whereupon the dried coating will be stripped from the casting surface by stripping forces applied to the paper or board backing. These considerations apply irrespective of whether the coating composition is first applied to the backing web and the coating surface of the web brought into intimate engagement with the coating drum, or whether the coating composition is initially applied to the drum and thereafter associated with the backing. In either event, it is essential that the coating be maintained in intimate non-slipping contact with the casting surface until the coating has set and that stripping occurs subsequent to the drying of the coating.

Having thus described the invention, and with the understanding that modifications may be made withoutdeparting from its spirit and purpose,-what we desire to secure and protect by Letters Patent is:

1. For use in the cast finishing of mineral coated paper and board, a mineral coating composition composed es sentially of a major proportion of mineral pigment, a minor proportion of an adhesive binder, and a small proportion of an emulsifiable polymer of ethylene, to-,

gether with an emulsifying agent for said polymer, said polymer being present in an amount on the order of 1 part polymer to parts coating composition, all on a dry basis.

2. For use in the manufacture of cast coated paper and board, a mineral coating composition consisting essentially of a major proportion of mineral pigment and a minor proportion of an adhesive binder, said composition having intimately admixed therein a small quantity of polyethylene emulsion on the order 2 to 12 parts emulsion for each 120 parts coating composition, on a dry weight basis.

3. The coating composition claimed in claim 2 wherein said adhesive binder comprises a polyvinyl acetate emulsion containing on the order of 20 parts solids for each 100 parts mineral pigment.

4. In a process for making cast mineral surfaced paper and board wherein a layer of aqueous coating composition consisting essentially of mineral pigment and an adhesive binder is applied to a moving heated casting surface and maintained in intimate contact therewith until said composition has dried, whereupon the dried layer is stripped from the casting surface in association with a web of backing material adhered to the side of said cast layer opposite said casting surface as an incident of the casting operation, the improvement which comprises intimately admixing with said aqueous coating composition prior to its application to said casting surface a small quantity of polyethylene on the order of about one-half parts polyethylene to each one hundred and twenty parts of the total coating composition, on a dry weight basis, the said polyethylene acting to render the dried coating composition essentially self-releasing from the casting surface.

5. In a process for making cast mineral surfaced paper and board wherein an aqueous coating composition consisting essentially of from 7090 parts mineral pigment and from 3010 parts adhesive binder, dry weight basis, is applied to a moving heated casting surface and maintained in intimate contact therewith until said composition has dried, whereupon the dried layer is stripped from the casting surface in association with a web of backing material adhered to the side of the cast layer opposite said casting surface as an incident of the casting operation, the improvement which comprises admixing with said aqueous coating composition prior to its application to the casting surface a quantity of polyethylene emulsion on the order of about 2 to 12 parts polyethylene emulsion for each one hundred twenty parts pigment and binder solids, said coating composition at the time of its application to the casting surface having an overall solid content of from 45-65% 6. The process claimed in claim 5 wherein said polyethylene emulsion comprises a low molecular weight polymer of ethylene having a molecular weight in the range of 2000-2500 and capable of being emulsified using anionic, cationic, and non-ionic emulsifying systems.

7. The process claimed in claim 6 wherein said adhesive binder comprises a vinyl acetate emulsion, and wherein said polyethylene emulsion contains polyoxyethylene lauryl ether as its principal emulsifying ingredient.

8. In a process for making cast mineral coated paper and board, the steps of compounding an aqueous coating composition consisting essentially of from 70 to 90 parts mineral pigment and from 30-10 parts adhesive binder, both on a dry weight basis, together with from 2-12 parts of a polyethylene emulsion for each one hundred and twenty parts pigment and binder solids, and adding sufficient aqueous vehicle to said composition to bring its overall solids content to from to applying a layer of said aqueous coating composition to a heated moving casting surface and maintaining said layer in intimate contact with said casting surface until it has dried, adhering a web of paper or board to the exposed surface of said dried layer and stripping the dried layer from said casting surface by means of said web, the polyethylene in said coating composition acting to render the dried coating composition essentially self-releasing from the casting surface.

9. The process claimed in claim 8 wherein the adhesive binder comprises a fine particle size polyvinyl acetate resin emulsion which is on the acidic side, and wherein said polyethylene emulsion is compatible with the acidic binder.

10. The process claimed in claim 8 wherein said polyethylene emulsion comprises alow molecular weight polymer of ethylene having a molecular weight in the range of 2000-2500, and wherein said polymer is capable of being emulsified using anionic, cationic and non-ionic emulsifying systems.

References Cited by the Examiner UNITED STATES PATENTS EARL M. BERGERT, Primary Examiner.

CARL F. KRAFFT, Examzner. 

4. IN A PROCESS FOR MAKING CAST MINERAL SURFACED PAPER AND BOARD WHEREIN A LAYER OF AQUEOUS COATING COMPOSITION CONSISTING ESSENTIALLY OF MINERAL PIGMENT AND AN ADHESIVE BINDER IS APPLIED TO A MOVING HEATED CASTING SURFACE AND MAINTAINED IN INTIMATE CONTACT THEREWITH UNITL SAID COMPOSITION HAS DIRED, WHEREUPON THE DRIED LAYER IS STRIPPED FROM THE CASTING SURFACE IN ASSOCIATION WITH A WEB OF BACKING MATERIAL ADHERED TO THE SIDE OF SAID CAST LAYER OPPOSITE SAID CASTING SURFACE AS AN INCIDENT OF THE CASTING OPERATION, THE IMPROVEMENT WHICH COMPRISES INTIMATELY ADMIXING WITH SAID QUEOUS COATING COMPOSITION PRIOR TO ITS APPLICATION TO SAID CASTING SURFACE A SMALL WAUNTITY OF POLYETHYLENE ON THE ORDER OF ABOUT ONE-HALF PARTS POLYETHYLENE TO EACH ONE HUNDERED AND TWENTY PARTS OF THE TOTAL COATING COMPOSITION, ON A DRY WEIGHT BASIS, THE SAID POLYETHYLENE ACTING TO RENDER THE DRIED COATING COMPOSITION ESSENTIALLY SELF-RELEASING FROM THE CASTING SURFACE. 